1. Field of the Invention
The present invention relates to an engine of a spark ignition type.
2. Description of the Related Art
Recently, there is the increasing tendency that an engine has a fuel injection valve of electronic type as a fuel supply means, which is arranged to be controlled with a control unit, for example, as disclosed in Japanese Patent Publication (kokoku) No. 2-36,772. The engines of this type can offer the advantage that an air-fuel ratio (a ratio of intake air to fuel to be admixed therewith) of a mixed fuel to be charged into the engine can be adjusted with high freedom.
Generally, an amount of a mixed fuel is supplied in such a manner that an air-fuel ratio of the mixed fuel is controlled so as to become stoichiometric (.lambda.=1 at A/F=14.7) when a running state of the engine exists in a medium load region, on the one hand, and that the air-fuel ratio of the mixed fuel is controlled so as to become rich as high as A/F=13 or less in order to attach much importance to output of the engine, when the running state of the engine exists in a high load region.
It can be noted, however, that making a air-fuel ratio rich in the high load region as in a conventional case suffers from the disadvantage that fuel economy becomes poor. It is thus preferred to control the air-fuel ratio so as to become stoichiometric even in the high load region when the fuel economy is of great account.
When the air-fuel ratio of the mixed fuel is set to become stoichiometric, combustion can be performed smoothly due to the existence of air in proper quantities as the air-fuel ratio of the mixed fuel becomes closer to the stoichiometric air-fuel ratio, this presents problems in that the temperature within the engine is elevated and a heat load becomes too high, for example, at a valve bridge between an intake valve and an exhaust valve.